This invention relates to a method of synthesis of the brookite phase of titanium dioxide.
Titanium dioxide is found in three crystallographic phases: the tetragonal rutile phase, the tetragonal anatase phase, and the orthorhombic brookite phase. The rutile phase is the stable phase, while the anatase and brookite phases are metastable with respect to rutile. Both anatase and rutile are readily synthesized at low temperature, but it is difficult to prepare pure brookite without appreciable amounts of anatase or rutile being mixed in with the brookite.
A wet chemical method for synthesizing brookite-based TiO2 has been reported by Ye and coworkers (X. Ye, J. Sha, Z. Jiao, and L. Zhang, “Thermoanalytical characteristic of nanocrystalline brookite-based titanium dioxide,” NanoStructured Materials, Vol. 8 (1997) pp. 919-927). The brookite TiO2 material was prepared in an ice-bath by using aqueous TiCl4 (0.03M), 2-ethyl hexanol (hydrocarbon), and sorbitan mono-oleate (surfactant) as the starting materials. Using ultrasonic agitation to accelerate the process, the microemulsified TiCl4 was precipitated as Ti(OH)4 by adding an aqueous NH3 solution (0.5M). Then the precipitated hydroxide was filtered and washed with pure water. Finally, to obtain the TiO2 powder, the precipitate was calcinated at 250° C. for 30 min. X-ray diffraction (XRD) analysis showed the product was brookite-based with a certain amount of anatase and rutile. Percentages of each phase were determined using XRD-phase-contrast analysis: brookite=40.8%, anatase=32.7%, rutile=26.5%. The authors report that, in the course of heating, there is a slow brookite-to-anatase transition below 780° C. Between 780 and 850° C., there is a rapid brookite-to-anatase transformation and a rapid anatase-to-rutile transition and rapid grain size growth. The brookite grain size formed at 250° C. was 15.5 nm, as determined by the XRD (121) peak.
Hydrothermal synthesis of brookite-type TiO2 nanocrystallites has been reported by Zheng Yanqing and coworkers (Zheng Yanqing, Shi Erwei, Li Wenjun, and Hu Xingfang, “Hydrothermal preparation and characterization of brookite-type TiO2 nanocrystallites,” J. Mater. Sci. Lett. Vol. 19 (2000) pp. 1445-1448). A solution NaOH was added dropwise to a solution of Ti(SO4)2 under stirring. The molar ratio of Ti(SO4)2 to NaOH was kept at 1:5. The resulting white precipitate was washed with water to remove SO42− and autoclaved with water as a reaction medium for hydrothermal reaction. Alternatively, a solution of NaOH is added to a solution of TiCl4 to adjust the pH of the solution to be greater than 8; a basic colloidal solution is formed, which is autoclaved. Hydrothermal reactions are run at temperatures of 200, 250, or 300° C. for 24 hours. With titanium chloride precursor, brookite phase is obtained at 200° C. and above. With titanium sulfate precursor, brookite phase is obtained at 250° C. and above.
Tang and coworkers have reported a synthesis of TiO2 nanoparticles that uses the low-temperature reaction of low-valent organometallic precursors. Bis(cyclooctatetraene)titanium reacts with dimethyl sulfoxide in organic solution at temperatures as low as room temperature to produce TiO2 In the absence of any supporting ligand, the reaction gives precipitation of amorphous TiO2 powder; in the presence of basic ligands such as tributylphosphine, tributylphosphine oxide and trioctylphosphine oxide, the precipitation is arrested, and chemically distinct, isolated, internally crystalline Tio2 nanoparticles are formed. Operations were conducted with the exclusion of air and water. (J. Ting, F. Redl, Y. Zhu, T. Siegrist, L. E. Brus, and M. L. Steigerwald, “An Organometallic Synthesis of TiO2 Nanoparticles,” Nano Letters vol. 5 (2005) pp. 543-548.)